In order to amplify an electric signal to a predetermined level, various amplifiers are used in an electric circuit or an electronic circuit. An amplifier among them, which is mainly used as a final stage amplifier of a transmitter and has large output power, is referred to as a power amplifier.
Since transmission output is increased in a radio or the like for performing communication over a wide range, the output power of the power amplifier used in the transmitter is also increased. Therefore, power consumed by this power amplifier becomes very large, thus occupying large percentage in power consumed by the entire transmitter. Accordingly, in order to reduce the power consumption of the transmitter, reduction of the power consumption of the power amplifier, i.e., increase of efficiency is an important development issue.
Generally, a transistor used in an amplifier is more efficient at near saturation as compared with when an output signal level is low, and that efficiency is called saturation efficiency. On the other hand, in recent wireless communication, there is a tendency to adopt an advanced digital modulation scheme, such as OFDM (Orthogonal Frequency-Division Multiplexing), for the purpose of efficient use of frequencies. For a signal obtained in such a scheme, a ratio of maximum power and average power (PAPR: Peak to Average Power Ratio) is very large. In a power amplifier, with regard to saturation power of a transistor to be used, it is necessary to consider the maximum power of a signal in order to suppress distortion of a signal waveform. Therefore, the power amplifier operates at the average power much smaller than the saturation power, and its efficiency tends to be very low with respect to the saturation efficiency.
Various methods have been devised as a way to achieve high efficiency of the power amplifier. Among them, a Doherty amplifier has started to be adopted as a power amplifier suitable for achieving high efficiency for a signal with large PAPR in power amplifiers for various uses in recent years.